A lampshade generally comprises a body or carcass (hereinafter referred to as a "frame") which constitutes a support for a fabric covering stretched against the elements of the frame and secured thereto. The frame generally comprises a lower circular element (lower circle), an upper circular element whose axis coincides with that of the lower circular element, and a plurality of angularly equispaced struts or armatures interconnecting these circular elements. In addition, the lampshade may be provided with additional struts extending inwardly from, for example, the upper circle and terminating, within the body of the lampshade, in an annular element by which the shade is mounted upon the lamp.
The principal difficulty encountered heretofore in the application of a fabric covering to such a body or frame has involved the stretching of the fabric against the element of the frame.
In one prior-art system, the hems at the upper and lower ends of the fabric covering are traversed by elastic members and these elastic members draw the fabric inwardly at the upper and lower ends against the upper and lower circles. The body is mounted on a support comprising a pair of axially spaced disks. This process, while permitting a certain degree of mechanization of the operation of stretching the fabric has several disadvantages. Firstly, the elastic can only be used for about a dozen times and the mounting of the elastic is relatively time consuming. Secondly, the stretching requires drawing the fabric first inwardly at the lower circle and then drawing the fabric inwardly at the upper circle, operations which are complex and also require considerable time.
Thirdly, the upper circle of the frame is frequently deformed by the stretching forces applied to the fabric and the deformations are especially pronounced when the lampshade is of large diameter.